there should be a chance to encounter the Mouth Fiend if you're wearing a ring gag and you've completed Cindy's quests. Maybe the Mouth Fiend heard tales of a hero with the perfect mouth, and tries to abduct them once seeing them in their ring gag.

Maybe if you encounter her in Town Square after escaping the first time, based on your dialog choices during the first abduction, the hero could expresses that they'll never really abandon their obligation to saving the world, but did secretely enjoy being locked in their dungeon for periods of time. The Mouth Fiend agrees and settles for something more "casual" between you two where you "roleplay" some recycled abduction scenes, letting the hero go after a week or so of fucking their face nonstop. 

Maybe even when after repeat encounters, depending on the Hero's Mouth Maniac Score, the Mouth Fiend will feel bold enough to take the hero to the nearest alley and fuck their throat before hauling them off to the dungeon for another session. 

Not many modifications would be needed for this variant, maybe an extra scene or two and dialog tweaks, but it offers an alternative for people who don't want to be barred from the brothel to access the Mouth Fiend or just want to have more access to Mouth Fiend content in their playthroughs.